It has been known for at least 30 years, since the discovery of Metglasses (iron based glass forming compositions used for transformer core applications) that iron based alloys could be made to be metallic glasses. However, with few exceptions, these iron based glassy alloys have had very poor glass forming ability and the amorphous state could only be produced at very high cooling rates (>106 K/s). Thus, these alloys may only be processed by techniques which give very rapid cooling such as drop impact or melt-spinning techniques.
While conventional steels have critical cooling rates for forming metallic glasses in the range of 109 K/s, special iron based metallic glass forming alloys have been developed having a critical cooling rate orders of magnitude lower than conventional steels. Some special alloys have been developed that may produce metallic glasses at cooling rates in the range of 104 to 105 K/s. Furthermore, some bulk glass forming alloys have critical cooling rates in the range of 100 to 102 K/s, however these alloys may employ rare or toxic alloying elements to increase glass forming ability, such as the addition of beryllium, which is highly toxic, or gallium, which is expensive. The development of glass forming alloys which are low cost and environmentally friendly has proven much more difficult.
In addition to the difficultly in developing cost effective and environmentally friendly alloys, the very high cooling rate required to produce metallic glass has limited the manufacturing techniques that are available for producing articles from metallic glass. The limited manufacturing techniques available have in turn limited the products that may be formed from metallic glasses, and the applications in which metal glasses may be used. Conventional techniques for processing steels from a molten state generally provide cooling rates on the order of 10−2 to 100 K/s. Special alloys that are more susceptible to forming metallic glasses, i.e., having reduced critical cooling rates on the order of 104 to 105 K/s, may not be processed using conventional techniques with such slow cooling rates and still produce metallic glasses. Even bulk glass forming alloys having critical cooling rates in the range of 100 to 102 K/s, are limited in the available processing techniques, and have the additional processing disadvantage in that they generally may not be processed in air but only under very high vacuum.
Common processing techniques used with metal glasses generally involve thermal spray coating. In a thermal spray coating process an atomized spray of molten metal may cool to a solid very quickly, exhibiting cooling rates in the range of 104 to 105 K/s. This rapid initial cooling facilitates the formation of a metallic glass structure. However, even while thermal spray coating may achieve a cooling rate sufficient to form metallic glass coatings, the rate of application of the coatings, as well as the coating thickness, may be limited by the need for secondary cooling of the solidified deposit down to room temperature. Secondary cooling may occur at much slower rate, typically in the range of 50 to 200 K/s. If a coating is too thick or the coating is built up too quickly, the thermal mass of the coating may cause devitrification, and the metallic glass coating may begin to crystallize.
Three methods that have been examined for producing an amorphous, or metallic glass, steel sheet or plate are spray forming, spray rolling, and planar flow casting followed by consolidation. Spray forming, such as spray casting, including the so-called Osprey process, involves depositing atomized liquid metal onto a substrate which collects and solidifies the droplets of the liquid metal. This method may be analogized to producing a thick cross-section by thermal spray coating.
Spray rolling is a method that is somewhat related to spray casting. Spray forming or casting may generally involve depositing atomized liquid metal on a substrate having a shape corresponding to the desired shape of the cast article. In the process of spray rolling, rather than spraying an atomized liquid metal onto a substrate, the atomized liquid metal may be sprayed onto two rollers. The rollers may compress the sprayed droplets to reduce the porosity of the accumulated droplets. Spray rolling may, therefore, produce a less porous and denser sheet than spray casting.
The third common method for producing sheets of steel metallic glass is planar flow cast ribbon consolidation. According to this method, thin ribbons of metallic glass may be produced using a planar flow method. Several thin ribbons may be stacked on top of one another to achieve a desired sheet or plate thickness. While the stacked metal ribbons are still in a heated condition they may be consolidated into a single sheet or plate by warm rolling. This process has generally been applied to minimize eddy current losses in amorphous transformer core alloys and has not been examined as a route to develop mechanical properties.